Description of problem:
Nova instances launch fail on node added post deployment with: "Host 'compute-2.localdomain' is not mapped to any cell" messages

Note: during scale out the following nodes are blacklisted:

parameter_defaults:
  DeploymentServerBlacklist:
    - compute-0
    - controller-0

Version-Release number of selected component (if applicable):
openstack-tripleo-heat-templates-9.0.1-0.20181013060864.ffbe879.el7ost.noarch

How reproducible:
100%

Steps to Reproduce:
1. Deploy with 3 controllers + 2 computes + 3 ceph nodes
2. Blacklist compute-0 and controller-0
3. Scale out with an additional compute node(compute-2)
4. Run tempest smoke test

Actual results:

Failures showing that the newly added node hasn't been mapped to any cell:
Details: {u'message': u"Host 'compute-2.localdomain' is not mapped to any cell", u'code': 400, u'created': u'2018-10-30T18:36:10Z'}


Expected results:
No failures, the newly added node should be mapped to the cell automatically.

Additional info:
It appears that these errors show up only when blacklisting controller-0.